Cardiologist too conservative?

I had to take a treadmill stress test yesterday because I had minor 
right-side chest pain a couple weeks after a 40% spontaneous pneumothorax
in 
my right lung. I suspect the test was overkill but I wasn't going to defy
my 
doc's advice.

He said he'd calculate 220 minus my age (49) so limited the test to 171
bpm. 
I told him that I sometimes approach 180 when exercising and that I'd like

to see if there are any problems when my heart reaches that range. Also
said 
I'd heard the 220-minus-age formula was arbitrary, He agreed but said it
was 
a good rule of thumb and summarily dismissed my request by pronouncing I 
just shouldn't exercise above my 220-minus-age. I realize that there is
some 
additional risk by exercising at higher heart rates but I'm willing to
take 
some measure of risk because I like to push myself and better my 
performance. Nonetheless he stopped me right at 171. I feel like the
doctor 
didn't do me a service because he didn't provide a sufficient reason for 
this and it seems his refusing to let me go further may have left him in
the 
dark about potential problems that could develop in these higher ranges if
I 
don't follow his advice.

I get the impression this cardiologist knows plenty about counseling and 
treating sedentary or ill patients but has no interest in advising fitter 
patients who want to push their endurance without undue risk. Has anyone
had 
experience with cardiologists who are more fitness-friendly? What have
they 
said about the risks of exercising at or near max HR?

BTW, it took me 15:30 to reach my "max" HR. He said his patients normally 
don't last longer than 11 minutes but heard of some accomplished athletes 
going over 20 minutes. I might've gone another minute or so but nowhere
near 
20 (with the speed/incline increases every minute). If you've done this 
test, how long did you go?